1. Field of the Invention
The present invention is directed to coupling and monitoring power of parallel arrays of light emitting device, more particularly for an array of vertical cavity surface emitting lasers (VCSELs), while also coupling the light from the light emitting devices into a corresponding array of fibers.
2. Description of Related Art
Light emitting devices such as VCSELs need some form of power control to maintain a constant output. Such control is typically automatically performed by measuring an output of a light emitting device during operation and using this measurement to control the power supplied to the light-emitting device.
Such control may be easily achieved when the light-emitting device to be controlled is an edge-emitting laser because edge emitting lasers output light from two ends thereof. Thus, one output may be used for the desired application, while the other output may be used for the power control.
In contrast, a VCSEL typically only emits light from one surface. Hence, any monitoring of the light must be from the same output as used for the desired application of the VCSEL. VCSELs are much cheaper and their surface emissions make them easier to integrate with other optical devices than the edge emitting lasers, so the use of VCSELs is very desirable.
Previous attempts to monitor the power of VCSELS typically involve splitting off of a portion of the output beam to use as a monitor beam. Examples of such configurations are disclosed in U.S. Pat. Nos. 5,757,836 and 5,774,486. However, such splitting off obscures part of the beam which may affect the wavefront and imaging, and hence coupling, of the light. Further, if the intensity distribution changes, such as when there is a change in lasing mode, the monitored power may change in a way which does not represent the overall output power of the VCSEL.
Additionally, splitting off of the beam may require the output of the VCSEL to be increased in order to maintain the requisite power level while allowing detection. Previous uses of scattering the beam to create a monitor beam relied on reflection for directing the beam and did not provide an optimal signal to the monitor detector. Further, previous scattering did not insure the entire beam was being monitored.
Further, light from light emitting devices used in a transmitter needs to be coupled to corresponding fibers. As the use of non-physical contact connections between light sources and fibers increases, the need for effective isolation to prevent light reflected at the fiber interface from being returned to the light source increases. Feedback to the light source may result in spectral broadening, light source instability, and relative intensity noise, which affect the monochromaticity of the light source. As data rates go up, the systems become more sensitive to relative intensity noise and require low bit error rates. Conventional optical isolators using polarization effects to attenuate reflection are very expensive, making the non-physical contact impractical. The importance of avoiding feedback is further increased when trying to use cheaper light sources, such as vertical cavity surfaces emitting laser diodes and light emitting diodes.
One solution that avoids the use of an optical isolator is a mode scrambler that divides power from the light source into many modes. A configuration employing a mode scrambler includes a single mode pigtail that provides light from the light source to the mode scrambler that then delivers the light to a transmission cable via an air-gap connector. Since any reflected power will still be divided across the many modes, any reflected power in the mode that can efficiently be coupled into the pigtail is only a small fraction of the total reflected power, thereby reducing return losses. However, this solution involves aligning another fiber, physically contacting the fiber with the mode scrambler, and placing the light source against the fiber. This pigtailing is expensive. Thus, there still exists a need for true nonphysical contact connection between a light source and a transmission system that does not require an isolator.
The present invention is therefore directed to power monitoring and coupling light from an array of light emitting devices that substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
The above and other objects may be realized by providing an integrated parallel transmitter including an array of light emitting devices, an optical system comprising at least two surfaces, a corresponding array of diffractive optical elements on one of the at least two surfaces splitting off a percentage of the light beam to create a monitor beam for each of the light emitting devices, another optical element on one of the at least two surfaces which performs further optical functioning on the monitor beam, a detector for measuring power of the monitor beam, and a corresponding plurality of couplers which couple remaining light not split off into a corresponding waveguide.
The optical system may further include performing at least one optical function to the functional light beam. The at least one optical function may include focusing the functional beam into a fiber. The diffractive optical element may be a transmission diffractive deflecting a percentage of the light beam to form a deflected beam. The another optical element may include a focusing optical element that focuses the monitor beam onto the detector. The integrated parallel transmitter may include metal on a surface opposite the transmission diffractive element, the metal reflecting the monitor beam. The light-emitting device may be a VCSEL array and the diffractive optical element and the detector are provided for each VCSEL in the VCSEL array. All elements of the optical system may be integrated onto a single substrate. The optical system may include at least three surfaces. The coupler may be a phase-matching coupler. The array of couplers may be formed on a same surface of the optical system and the array of diffractive optical elements may be formed on a same surface of the optical system. The light emitting devices and the power monitors may be mounted on a bottom surface of the optical system or may be mounted on a substrate separate from the optical system.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.